1. Field of the Invention
This invention relates to industrial process control systems for processes which are spread out over relatively large areas, yet are to be controlled from a central operating station. More particularly, this invention relates to an improved control system configuration which is related to the physical layout of the process so as to provide for efficient transfer of data and control instructions between the central station and the remote regions of the process, including data from sensing elements and instructions for operating elements such as actuators.
2. Description of the Prior Art
A considerable number of process control systems have been exposed and described in the art, and many have gone into commercial use. In more recent years, digital computers have been extensively used with process control systems to monitor functioning of the process and to direct the overall functioning of the control system. Such computer-directed systems have generally been classified as (1) supervisory control systems, wherein the computer develops set-point signals for individual controllers associated with each process valve, or (2) direct digital control systems, wherein the computer develops valve-control signals to be sent directly to valve-positioning equipment for each valve. Selected prior art disclosures of typical systems may be found in U.S. Pat. No. 3,582,621 (Lawler), U.S. Pat. No. 3,761,889 (Halley et al), and U.S. Pat. No. 3,971,000 (Cromwell).
The earlier computer-control systems generally used conventional solid wire conductors for transferring data, e.g. in the form of DC signals, between the central station and the system elements in the field. Thus, electrical conductors in heavy shields were led from the sensing elements, valves, etc., to junction boxes, and from there to central or other processors. For relatively large systems, the cost of such conventional wired data transfer networks has become very great, because they are expensive to install and involve high costs of planning, layout, and documentation. Improvements have been effected by transmitting the data part of the way in digital format through cables which serve a number of process elements, as disclosed in U.S. Pat. No. 3,760,374 (Nabi). But nevertheless it is clear that further improvements are needed.